soc: ixp4xx: Add DT bindings for IXP4xx NPE

This adds DT bindings for the Intel IXP4xx Network Processing
Engine (NPE). These are three firmware-executing units found in
the IXP4xx SoC. These bindings use YAML.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>